Ovulation is the process where an ovum or ova are released from the ovaries. The timing of ovulation within the menstrual cycle is of foremost importance for fertilization. It is well recognized that follicles acquire the ability to ovulate following growth and maturation stimulated by the pituitary gonadotropins. Follicle stimulating hormone (FSH) is predominantly responsible for follicular growth and luteinizing hormone (LH) stimulates ovulation. This coordinated process of gonadotropin-stimulated maturation of the follicle ensures delivery of a competent ova at ovulation. The adequately prepared ovum is then available for fertilization by sperm within hours after ovulation.
Ovulation is a finely timed process that is driven by pituitary gonadotropin stimulation of the ovary, and modified by the growth and biochemical (e.g., steroidogenic, inhibin secretion, etc.) response of follicles to the gonadotropin stimulation. During the normal menstrual cycle in women these hormones exhibit cyclic patterns. The menstrual cycle can be functionally divided into three phases: the follicular, the ovulatory and luteal phases. The follicular period begins at the end of the luteal phase of the preceding non-conceptive cycle, prior to or coincident with the onset of menses. The cycle starts with a transient rise in blood levels of FSH that stimulates development of a cohort of ovarian follicles. The size of the follicles recruited to grow is about 5 mm in diameter. In a natural menstrual cycle, usually one large or dominant follicle is established during the follicular phase, and it is committed to growth to maturation. In humans, the size of the follicle that is considered ready to ovulate is about 15 mm or more in diameter.
The second critical event that occurs in the ovary during the follicular phase is that granulosa cells within the ovarian follicles acquire receptors for LH and become increasingly responsive to LH. Secretion of estradiol and estrone from the ovary increases slowly at first, in parallel to the increasing diameter of the follicle and sensitivity of the follicle to LH. The relatively rising levels of estrogen and inhibin cause inhibition of gonadotropin releasing hormone (GnRH) secretion from the hypothalamus and gonadotropin secretion from the pituitary. Estrogen production reaches a maximum on the day before the LH peak and the neuroendocrine response to increased estrogen and gradually increasing concentrations of progesterone is the preovulatory release of gonadotropins which is discussed below.
During the ovulatory phase there is a change in the neuroendocrine response to estrogen and progesterone. At this point in the cycle, elevated estrogen elicits a preovulatory surge in serum FSH and LH levels, due to a positive feedback on the hypothalamus, estrogen now stimulating a surge in the levels of GnRH and subsequently FSH and LH release from the pituitary. This surge of gonadotropins induces the completion of follicular maturation and causes rupture of the dominant or Graafian follicle and discharge of the ovum some 16 to 24 hours after the LH peak. During the period following the preovulatory surge, serum estradiol levels temporarily decline and plasma progesterone levels begin to rise.
Following ovulation, the post-ovulatory ovarian follicle cells under the influence of LH are luteinized to form a corpus luteum—the luteal phase. The diagnostic markers of the luteal phase of the menstrual cycle are the marked increase in progesterone secretion by the corpus luteum, and the uterine transformation that occurs in response to progesterone. Associated with luteal progesterone production, there is a less pronounced increase in serum estrogen levels. As progesterone and estrogens increase, LH and FSH decline throughout most of the luteal phase. Towards the end of the luteal phase, in a non-conceptive menstrual cycle, the corpus luteum regresses and serum FSH levels begin to rise to initiate follicular growth for the next cycle.
FSH and LH are distinguished from each other by their ability to stimulate follicular development or ovulation, respectively. Both agents are known to stimulate an increase in intracellular cAMP concentrations. Agents that mimic cAMP such as forskolin or stable analogs of cAMP have been shown, in vitro, to resemble the effects of FSH in granulosa cells from immature follicles, and to resemble the effects of LH in cells from mature follicles. Although alternative intracellular pathways have been proposed for both FSH and LH, it is well accepted that cAMP is stimulated in response to both gonadotropins. If and when elevations in cAMP levels are associated with follicular development and maturation or ovulation induction depends on the cell types and the presence or absence of the respective receptors. Indeed, it has been demonstrated that mice which are deficient in a particular phosphodiesterase have impaired ovulation and diminished sensitivity of granulosa cells to gonadotropins.
Infertility treatments currently in clinical use incorporate some of the regulatory events described above. One agent which stimulates follicular growth and is used for treatment of anovulation is clomiphene. Clomiphene is a nonsteroidal antiestrogen that competes for estrogens at their binding sites. It is thought that clomiphene binds to estrogen receptors in the hypothalamus and pituitary and blocks the negative feedback exerted by ovarian estrogens. The result is increased output of gonadotropins (FSH and LH) during the early part of the follicular phase. The effect of clomiphene is to increase endogenous FSH serum levels and to improve the growth and maturation of follicles. Subsequently either endogenous LH or exogenous LH/CG induce ovulation in these patients.
In addition to clomiphene, women have been treated with ovulation induction regimens which include commercial preparations of the human gonadotropins, including follicle stimulating hormone (FSH) and luteinizing hormone (LH) or chorionic gonadotropin (CG), all of which were first obtained by purification of urine from pregnant women and more recently by recombinant technology. In general, this treatment is highly effective in stimulating folliculogenesis and steroidogenesis. Complications of this treatment result from the fact that these preparations and regimens can over-stimulate follicular development and maturation of follicles. In a subset of patients, the ovary can become hyperstimulated, which may result in multiple ovulations and, consequently, multiple births. Not only can ovarian hyperstimulation be life threatening to the mother, it also typically results in newborns with lower birth weight, who subsequently require intensive care. It is believed that the principal complications attributed to gonadotropin-induced hyperstimulation and multiple pregnancies probably result from the prolonged effects of human chorionic gonadotropin (hCG). In addition, use of gonadotropins in ovulation induction regimens can result in injection site reactions, both local and systemic. Consequently, the development of ovulation induction regimens using orally active agents with milder gonadotropin-like activity as opposed to therapies that use potent injectables would be of substantial benefit. More importantly, it would be a significant advantage if ovulation induction regimens could be developed which result in less ovarian hyperstimulation and, consequently, present less danger to the mother and produce healthier newborns.